As a hydraulic drive system mounted on a construction machine to perform combined operations of plural hydraulic cylinders, many techniques have been proposed to date (for example, JP-A-2000-337307).
FIG. 11 is a hydraulic circuit diagram showing the construction of an essential part of a hydraulic drive system arranged in this kind of conventional techniques, and FIG. 12 is a side view illustrating a hydraulic excavator on which the hydraulic drive system shown in FIG. 11 is arranged.
The hydraulic excavator illustrated in FIG. 12 is provided with a travel base 1, a swing superstructure 2 arranged on the travel base 1, a boom 3 mounted pivotally in a vertical direction on the swing superstructure 2, an arm 4 mounted pivotally in a vertical direction on the boom 3, and a bucket 5 mounted pivotally in a vertical direction on the arm 4. The boom 3, arm 4 and bucket 5 make up front attachments. The hydraulic excavator is also provided with a boom cylinder 6 which constitutes a first hydraulic cylinder for driving the boom 3, an arm cylinder 7 which constitutes a second hydraulic cylinder for driving the arm 4, and a bucket cylinder 8 for driving the bucket 5.
FIG. 11 shows a center-bypass hydraulic drive system for driving the boom cylinder 6 and arm cylinder 7 in the above-mentioned hydraulic drive systems suitable for arrangement on hydraulic excavators.
As shown in FIG. 11, the boom cylinder 6 is provided with a bottom chamber 6a and a rod chamber 6b. By feeding pressure oil to the bottom chamber 6a, the boom cylinder 6 is caused to extend to perform boom raising. By feeding pressure oil to the rod chamber 6b, on the other hand, the boom cylinder 6 is caused to retract to perform boom lowering. The arm cylinder 7 is also provided with a bottom chamber 7a and rod chamber 7b. By feeding pressure oil to the bottom chamber 8a, arm crowding is performed. By feeding pressure oil to the rod chamber 7b, on the other hand, arm dumping is performed.
The hydraulic drive system which includes these boom cylinder 6 and arm cylinder 7 is provided with an engine 20, a main hydraulic pump 21 driven by the engine 20, a directional control valve 23 for the boom as a first directional control valve for controlling a flow of pressure oil to be fed from the main hydraulic pump 21 to the boom cylinder 6, an directional control valve 24 for the arm as a second directional control valve for controlling a flow of pressure oil to be fed from the main hydraulic pump 21 to the arm cylinder 7, a boom control device 25 as a first control device for selectively controlling the directional control valve 23 for the boom, an arm control device 26 as a second control device for selectively controlling the directional control valve 24 for the arm, and a pilot pump 22 driven by the engine 20.
The directional control valve 23 for the boom is arranged on a line 28 extending to a delivery line of the main hydraulic pump 21, while the directional control valve 24 for the arm is arranged on a line 27 extending to the above-mentioned delivery line.
The directional control valve 23 for the boom and the bottom chamber 6a of the boom cylinder 6 are connected via a main line 29a, while the directional control valve 23 for the boom and the rod chamber 6b of the boom cylinder 6 are connected via a main line 29b. Similarly, the directional control valve 24 for the arm and the bottom chamber 7a of the arm cylinder 7 are connected via a main line 30a, while the directional control valve 24 for the arm and the rod chamber 7b of the arm cylinder 7 are connected via a main line 30b. 
The boom control device 25 is connected to the pilot pump 22. A pilot pressure produced as a result of its operation is fed via one of pilot lines 25a,25b to a corresponding control chamber of the directional control valve 23 for the boom such that the directional control device 23 for the boom is changed over into the left position or the right position as viewed in FIG. 11. Similarly, the arm control device 26 is also connected to the pilot pump 22. A pilot pressure produced corresponding to a control stroke is fed via one of pilot lines 26a,26b to a corresponding control chamber of the directional control valve 24 for the arm such that the directional control device 24 for the arm is changed over into the left position or the right position as viewed in FIG. 11.
In the hydraulic excavator provided with the hydraulic drive system constructed as described above, the boom control device 25 shown in FIG. 11 is controlled upon performing digging or the like of earth, and a pilot pressure is hence produced, for example, in the pilot line 25a. When the directional control valve 23 for the boom is changed over into the left position as viewed in FIG. 11, the pressure oil delivered from the main hydraulic pump 21 is fed to the bottom chamber 6a of the boom cylinder 6 via the line 28, the directional control valve 23 for the boom and the main line 29a, while the pressure oil in the rod chamber 6b is caused to return to a reservoir 43 via the main line 29b and the directional control valve 23 for the boom. As a result, the boom cylinder 6 extends as indicated by arrow 13 in FIG. 12 so that the boom 3 is pivoted as indicated by arrow 12 in FIG. 12 to perform boom raising.
Concurrently with this boom raising operation, the arm control device 26 is also controlled and a pilot pressure is hence produced, for example, in the pilot line 26a. when the directional control valve 24 for the arm is changed over into the left position as viewed in FIG. 11, the pressure oil delivered from the main hydraulic pump 21 is fed to the bottom chamber 7a of the arm cylinder 7 via the line 27, the directional control valve 24 for the arm and the main line 30a, while the pressure oil in the rod chamber 7b is caused to return to the reservoir 43 via the main line 30b and the directional control valve 24 for the arm. As a result, the arm cylinder 7 extends as indicated by arrow 9 in FIG. 12 so that the arm 4 is pivoted as indicated by arrow 11 in FIG. 12 to perform arm crowding.
When an unillustrated bucket control device is also controlled concurrently with such a boom raising and arm crowding operation to change over a directional control valve for the bucket such that the bucket cylinder 8 illustrated in FIG. 12 is caused to extend in the direction of arrow 10 in FIG. 12, the bucket 5 is caused to pivot in the direction of arrow 11 to perform earth digging work or the like as desired.
FIG. 13 contains characteristic diagrams illustrating pilot pressure characteristics and cylinder pressure characteristics in the above-described combined operation. In the lower diagram of FIG. 13, the time length of digging work is plotted along abscissas, and the pilot pressure produced by the control device is plotted along ordinates. Numeral 31 in the lower diagram of FIG. 13 indicates pilot pressures produced by the arm control device 26 and to be fed to the pilot line 26a, while numeral 32 in the lower diagram of FIG. 13 designates pilot pressures produced by the boom control device 25 and to be fed to the pilot line 25a, that is, pilot pressures upon boom raising. T1, T2 and T3 indicate time points at which boom raising operations were performed, respectively.
In the upper diagram of FIG. 13, on the other hand, time lengths of digging work are plotted along abscissas, and load pressures produced in the hydraulic cylinders 6, 7, in other words, cylinder pressures are plotted along ordinates. Numeral 33 in the upper diagram of FIG. 13 indicates bottom pressures produced in the bottom chamber 7a of the arm cylinder 7, that is, arm cylinder bottom pressures, while numeral 34 designates rod pressures produced in the rod chamber 6b of the boom cylinder 6, that is, boom cylinder rod pressures. When such a combined operation of boom raising and arm crowding is performed, force in the direction of arrow 12 in FIG. 12 is transmitted to the boom 3 by counterforce produced when the bucket 5 digs earth. As a consequence, the boom cylinder 6 tends to be pulled in the direction of arrow 13 in FIG. 12, and as indicated by the boom rod pressure 34 in the upper diagram of FIG. 13, a high pressure is produced in the rod chamber 6b of the boom cylinder 6.